Fabrication of diaphragms and &#34;floating&#34; regions of single crystal semiconductor for MEMS devices

ABSTRACT

A single crystal semiconductor region is fabricated in a semiconductor wafer. The region is either cantilevered, supported at one or both ends, or midpoint, or supported at multiple locations. After a pattern and etch step, a dielectric fill step is performed to define the boundaries of the region in the semiconductor wafer. Oxygen or nitrogen is implanted in the semiconductor wafer on a surface area of the semiconductor wafer that corresponds to a top surface of the region. The annealing of the oxygen or nitrogen ions convert the silicon to an oxide or a nitride beneath the surface area. The silicon dioxide or silicon nitride is etched away to produce a semiconducting region of a single crystal material.

BACKGROUND OF THE INVENTION

This invention relates to Microelectromechanical Structure, or MEMS, andthe method of fabricating such devices.

Microelectromechanical Structures, or “MEMS” often consist in part, ofpolycrystalline semiconductor regions that are only partially attachedto the underlying material. Such polycrystalline silicon regions, whichmay be cantilevered, center supported or supported at both ends arehereinafter referred to as “regions”. They are typically formed bydepositing polysilicon on a layer of sacrificial material such as asilicon nitride, silicon dioxide, or another dielectric. Often thepolysilicon material, either doped or undoped as required by the design,is masked and patterned using a photolithographic process to define thedesired structure(s). The underlying sacrificial layer is partially ortotally removed, leaving the layer of polysilicon material unsupportedexcept at selected support locations. This sequence of steps allows thefabrication of structures such as beams, which might be supported at oneor both ends as well as diaphragms and other shapes and structures thatare characterized by only being supported at selected points. In someMEMS, the polysilicon structure that is formed by the removal of thesacrificial layer may not be permanently attached, but may beconstrained, allowing it even greater freedom. The use of polysilicon issufficient in some applications, but the strength of polysilicon and itsresistance to crack formation and associated mechanical failure are notas high as those of single crystal materials.

One prior art method for forming these regions involves depositing anetch stop layer of silicon nitride in direct contact with or above thesemiconductor material substrate. Next a sacrificial layer of silicondioxide is deposited on top of the etch stop layer of silicon nitride. Aregion of polysilicon is then deposited, masked, patterned and etched onthe layer of sacrificial material. The underlying sacrificial layer ofsilicon dioxide is partially or totally removed, leaving the layer ofpolysilicon material “floating” and unsupported except at somelocations.

The mechanical properties of these floating regions may become moreimportant with the continuing miniaturization of integrated circuits.The ever increasing complexity of integrated devices having greaternumbers of wiring channels coupled with the desire for packing chipscloser together to minimize transmission delays, results in the need formultilayered and high channel density interconnect substrates. Also,with higher interconnect wiring density, the need for using insulatorswith low dielectric constants becomes more important for performancereasons. Insulators with the low dielectric constants include vacuum,gases such as air, and depending upon the temperature, singlecrystalline silicon. The present invention is also directed tofabricating interconnect substrates with air as the dielectric in aprocess which allows the formation of floating single crystalstructures.

FIG. 1, to which reference should now be made, illustrates arepresentation of the sequence of steps used to manufacture the priorart microelectromechanical (MEMS) devices.

Starting with a semiconductor material substrate 12, in FIG. 1 a, anetch stop layer 15 of silicon nitride is deposited in direct contactwith the substrate, and then a sacrificial layer 1 of silicon dioxide isdeposited on top of the etch stop layer 15 of silicon nitride.

Referring to FIG. 1 b, A region of polysilicon 4 is then deposited,masked, patterned and etched on the layer of sacrificial material 1.

After the step illustrated in FIG. 1 b, as shown in FIG. 1 c, theunderlying sacrificial layer 1 of silicon dioxide is partially ortotally removed which leaves the layer of polysilicon material 4“floating” and unsupported at some locations 5.

In a related area of art, air bridges are growing in importance with theadvancement of speed requirements. An example of air bridges in five (5)layers is disclosed in U.S. Pat. No. 4,920,639 dated May 1, 1990, whichis hereby incorporated by reference, describes “a method of building amultilevel electrical interconnect supported by metal pillars with airas a dielectric. The metal conductors and metal support pillars areformed using a photo-imagible polymer which serves the function ofpatterning and also provides a temporary support during construction.”

Another useful invention is described in U.S. Pat. No. 5,891,797 datedApr. 6, 1999, which is hereby incorporated by reference, and states that“a process of manufacturing integrated circuits is disclosed fordesigning and implementing a hierarchical wiring system. Theinterconnection requirements are sorted and designed into a particularwiring level according to length. Support structures may be constructedto allow more flexibility in designing air bridge dimensions. Thesupport structures may take the form of lateral ribs or intermediateposts, and may be fabricated of either insulating or conductivematerial. One integrated circuit described is a memory device, such as adynamic random access memory.”

U.S. Pat. No. 6,020,215 dated Feb. 1, 2000, which is hereby incorporatedby reference, and describes “a microstructure comprising a substrate(1), a patterned structure (beam member) (2) suspended over thesubstrate (1) with an air-space (4) there between and supportingstructure (3) for suspending the patterned structure (2) over thesubstrate (1). The microstructure is prepared by using a sacrificiallayer (7) which is removed to form the space between the substrate (1)and the patterned structure (2) adhered to the sacrificial layer. In thecase of using resin as the material of the sacrificial layer, thesacrificial layer can be removed without causing sticking, and anelectrode can be provided on the patterned structure. The microstructurecan have application as electrostatic actuator, etc., depending onchoice of shape and composition.”

The assembly of air bridges require inter-connecting surfaces as shownU.S. Pat. No. 6,060,381 dated May 9, 2000, which is hereby incorporatedby reference and describes “an electronic part having an air-bridgeinterconnection with a flat air-bridge interconnection body, nointerconnection loss, high Q and low power consumption. Also disclosedis a method of manufacturing such electronic parts. The flat air-bridgeinterconnection body is obtained by conducting two-stage selectiveplating including selective plating for forming posts on post baseelectrodes and selective plating for forming the air-bridgeinterconnection body.”

Thus, it can be shown that there is a need for beams or bridges that areonly partially supported. These beams have application in themicroelectronical structure art as well as in the air bridge art.

SUMMARY OF INVENTION

A single crystal semiconductor region is fabricated in a semiconductorwafer. The region may be cantilevered, supported at both ends, orsupported at points along its length. An etch and oxide fill step or anoxide formation step is performed to define the boundaries of the regionin the semiconductor wafer. Oxygen is implanted into the semiconductorwafer beneath a surface area of the semiconductor wafer that correspondsto a top surface of the beam and at least part of the surrounding oxideboundary region. Annealed oxygen ions convert the semiconductor materialbeneath the surface area to silicon dioxide. The silicon dioxide isetched away to produce a semiconducting region of a single crystalmaterial.

This invention disclosure describes a technique for fabricatingMicroelectromechanical (MEMS) devices, and other semiconductor beam typestructures, which allow the formation of single crystal beams instead ofpolysilicon regions. The method includes depositing and defining an etchstop layer on the surface of a substrate covering all areas that willnot become a single crystal region. The etch stop layer can be composedof materials selected from a group which would include silicon nitride(Si₃N₄) and doped silicon dioxide (SiO₂). Photolithography, trench etch,and oxide fill steps are performed to define the shape of the singlecrystal region. A blocking layer of photoresist or other material isthen deposited and defined using a photolithography process on thesurface of the etch stop layer to prevent the ion implantation of oxygenwhere it is not desired. The single crystal structure is formed by ahigh dose implantation of oxygen to begin to convert the silicon orother material to an oxide below the surface of the single crystalmaterial. An anneal operation is performed to complete the conversion ofthe material to oxide. Both the oxide region surrounding the singlecrystal material on its sides and the oxide layer formed by implantationof oxygen below the surface are etched away, which produces acantilevered beam or similar structure of single crystal semiconductormaterial. Nitrogen may be substituted for oxygen in the implantationstep, and the trench may be filled with silicon nitride in anotherversion.

In another embodiment of the invention, overlapping regions of singlecrystal silicon are formed by repeating and modifying the process stepsthat are shown in the FIG. 2.

Another embodiment could include epitaxially depositing additionalsingle crystal material to increase the thickness of the single crystalregion, thereby creating an air bridge structure of single crystalsemiconductor material.

Yet another embodiment of the invention fabricates multiple layers ofsingle crystal silicon by using similar steps to those in FIGS. 2 and 3,but including the deposition of metal in via's, implanting oxygen,etching away the oxide region leaving a single crystal semiconductormaterial air bridge supported by metal posts. For these applications, arefractory metal such as tungsten, which can withstand high temperaturesand can also form ohmic contacts to semiconductors may be used ifelectrical contact to the single crystal bridge(s) is desired, and hightemperature processing is to occur following formation of the metalposts. Otherwise aluminum, its alloys, or other metals could be used.

An embodiment of this invention that does not require the step ofcreating a dielectric border is shown in FIG. 6. In this process flow,the temporary region of dielectric is formed first, and a trench etchoccurs next, extending from the surface to the temporary region ofdielectric. The geometry of the trench etch determines the number ofpoints at which the floating region of single crystal silicon isattached to the substrate and the size of these attachment points. Afterthe trench etch is completed, an etchant such as buffered oxide etch(BOE) is used to remove the temporary region of dielectric.

The thickness of the diaphragm shown in FIG. 6 is limited by theimplantation range of species that is used to form the temporary regionof dielectric. This limitation may be overcome by depositing anepitaxial layer of silicon (or other material) after the step ofimplanting the species to form the temporary region of dielectric, butbefore the trench etch step. This process sequence is shown in FIG. 7.

The practice of this invention can also create DMD (Deformed MirrorDevice) and air bridge devices with an accurate and simple technique.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a prior art process showing the use of a sacrificiallayer and polycrystalline material to fabricate a cantilevered region;

FIG. 2 illustrates steps used to fabricate floating structures of singlecrystal silicon;

FIG. 3 illustrates the results of a repetitive process used in FIG. 2 tofabricate multiple layers of floating or cantilevered structures ofsingle crystal silicon;

FIG. 4 illustrates an air bridge of one layer of single crystal siliconwith metal posts supporting both ends that have been fabricated usingthe teachings of this disclosure;

FIG. 5 illustrates an air bridge of multiple layers of single crystalsilicon with metal posts supporting both ends that have been fabricatedusing the teachings of this disclosure;

FIG. 6 illustrates the formation of the single crystal region using atrench etch step after ion implantation; and

FIG. 7 illustrates the growth of an epitaxial layer following ionimplantation to obtain a thicker single crystal region.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 2, to which reference should now be made, illustrates arepresentation of the sequence of steps used to manufacturemicroelectromechanical (MEMS) devices with a process that allows theformation of floating or cantilevered single crystal structures insteadof the polysilicon structures used in the prior art.

In FIG. 2 a, an etch stop layer 15 such as silicon nitride is depositedover existing structures 11 using a vapor deposition process typicallyin the temperature range of 800° C.-900° C., and then masked and etched,leaving the nitride in direct contact with the surface of a singlecrystalline substrate at the boundaries of all regions that will notbecome floating structures, leaving regions that will become floatingstructures in between

In FIG. 2 b, using a thermal oxidation furnace in a temperature range of800° C.-1250° C., perform a local oxidation of silicon (LOCOS) processsequence after a trench etch step. This process sequence can include anyof the known variations of polysilicon buffered LOCOS, side wall maskisolation (SWAMI), or trench etch and refill, using a plasma reactor.Following this process sequence, and a subsequent mask and etch step toremove unwanted nitride, an oxide filled region 20 defines theboundaries of the single crystal structure. Alternately 20 may consistof CVD silicon nitride that has been etched to leave the nitride in justthe trenches.

In FIG. 2 c, a blocking layer 25 of photo resist or another material isapplied using a metered resist dispenser and spin technique. Aphotolithography process step is used on the surface of the photoresistto define the area for the ion implantation of oxygen or nitrogen.

In FIG. 2 d, a high dose oxygen or nitrogen ion implant 30, normally inthe range of 10¹⁷-10¹⁸ ions/cm² utilizing a high current (20 mA-100 mA)implant machine is performed to begin to convert the silicon to an oxideor nitride below the surface of the previously defined area of thesubstrate of single crystal material. The blocking layer is used to etchthe center nitride region and then the blocking layer is removed using awet chemical or a dry plasma etching process, though in someapplications, the silicon nitride may be left on the center region.

In FIG. 2 e, an anneal process 35 is performed at a temperature above600° C. using a process from a group which includes a furnace annealover a period of many minutes and rapid thermal anneal (RTA) over a fewseconds to complete the conversion of the implanted material to an oxideor a nitride.

In FIG. 2 f, both the oxide region surrounding the single crystalsemiconductor region and the oxide or nitride layer formed by ionimplanting and annealing the oxygen or nitrogen beneath the surface ofsingle crystal semiconductor material substrate 12 are etched away 40,using either a wet etch or a dry plasma etching process. This produces afloating structure or a structure that is connected to the originalsemiconductor substrate at one or more locations that is of singlecrystal semiconductor material 45.

While FIG. 2 has the oxygen or nitrogen implant and anneal step (FIG. 2d) before the trench formation step (FIG. 2 c), the order of these twosteps could be reversed while maintaining the ability to form the samestructures of floating or cantilevered single crystal semiconductormaterial. The thickness of the resulting single crystal structure couldalso be increased by selective epitaxial growth or epitaxial and polydeposits followed by an etch back step.

FIG. 3, to which reference should now be made, illustrates the sequenceof steps used to manufacture microelectromechanical (MEMS) devices witha process which allows the formation of floating or cantilevered singlecrystal silicon structures at different depths into the substrate.

FIG. 3 a shows the substrate following an oxide formation step toproduce oxide layer 51, masking and etch steps to produce opening 53,and an ion implantation step using oxygen or nitrogen as the implantedspecies to produce region 60 a. A high dose oxygen or nitrogen ionimplantation, normally in the range of 10¹⁷-10¹⁸ ions/cm² utilizing ahigh current (20 mA-100 mA) is performed to begin to convert thesemiconductor to an oxide or nitride layer below the surface of thesingle crystal substrate. A thin layer of oxide may then be grown, whichmay also anneal the implanted oxygen or nitrogen to form a dielectriclayer, while simultaneously forming a step in the surface of thesubstrate, which allows for the alignment of subsequent masking layers.The oxide blocking layer and any additional thermally grown oxide areremoved, and a layer of epitaxial semiconductor 60 is deposited over theentire wafer. Next a layer of oxide 61 is formed, masked, and etched,and a second oxygen or nitrogen ion implantation step is performedthrough opening 63 as shown in FIG. 3 b to form region 60 b, which isshown being above region 60 a in this figure. Region 60 b may be in anyrelative position to region 60 a, allowing the formation ofsemiconductor regions more complex than just identical stacked regions.

In FIG. 3 c, as was illustrated in FIG. 2 a, etch stop layer 65 such assilicon nitride is deposited, using a vapor phase process in thetemperature range of 800° C.-900° C., in direct contact with the surfaceof the substrate at the boundaries of all regions that will not becomefloating structures. Using a thermal oxidation furnace in a temperaturerange of 800° C.-1250° C., to perform a local oxidation silicon (LOCOS)step, after a trench etch step which may include any variations ofpolysilicon buffered LOCOS, side wall mask isolation (SWAMI), and trenchetch, using a plasma reactor is performed with an oxide fill step 70 todefine the shape of the surface boundaries of the single crystalstructure. Alternately, 70 may consist of CVD silicon nitride that hasbeen etched to leave the nitride in just the trenches.

In FIG. 3 d, both the oxide or nitride regions surrounding the singlecrystal semiconductor regions and the oxide or nitride layers formed byion implanting the oxygen or nitrogen beneath the single crystalsemiconductor material are etched away 90, using a wet etch or a dryplasma etching process. The masking process used for etching inconjunction with the etch rates between the oxide or nitride and thesemiconductor material result in corner supports for the region ofsingle crystal semiconductor material. This results in the formation ofthe first and second single crystal silicon structures 80 a and 80 b.

Although only two vertical structures are shown in FIG. 3, by growingadditional epitaxial layers and performing ion implants, many verticalstructures 80 may be fabricated. Additionally the process may also beused to fabricate multiple structures on a single epitaxial layer.

The single crystal region(s) shown in FIGS. 2 and 3 may also besupported and electrically contacted by one or more electricallyconductive posts 100 that are incorporated into the structure as shownin FIGS. 4 and 5 prior to the etch stip that removes the sacrificiallayer from around the single crystal region(s). The metal posts mayconsist of any conductor such as aluminum, copper or tungsten. If nofurther high temperature steps are to be performed, aluminum or copperare appropriate. However, if further high temperature steps arerequired, a refractory metal such as tungsten should be used. The metalposts are similar to the metal “plugs” used to connect conductive leadson two different metal layers and would be formed in the same fashion.First, one or more openings are etched using conventionalphotolithographic patterning and etching techniques. Next, theelectrically conductive post(s) are formed using deposition or platingtechniques followed by an etching or polishing step (if needed) to leavejust the post(s). These conductive posts may be fabricated from a listof materials including metals, alloys of metals, silicides and dopedpolycrystalline semiconductors.

FIG. 6 shows an embodiment of this invention that does not require thecreation of a dielectric border to define the boundaries of singlecrystal diaphragm or floating structure. The temporary region ofdielectric 110 is typically formed by implanting oxygen or nitrogen andperforming a high temperature anneal step. Subsequent to the formationof the temporary region of dielectric, one or more trenches 115 areformed above part or all of the temporary region of dielectric. Thetemporary region of dielectric is then removed, forming the singlecrystal diaphragm or floating structure.

FIG. 7 shows an embodiment of the invention that allows the formation ofsingle crystal diaphragms or floating structures that are thicker thanthose that can be obtained using just ion implantation. An epitaxiallayer of single crystal semiconductor 120 is formed after the formationof the temporary regions of dielectric 110, but before etching thetrenches 115.

1. A method for fabricating a semiconductor region having a singlecrystal structure that is supported at one or more locations, comprisingthe steps of: masking the first surface according to a predeterminedshape to create an implant area on the first surface; implanting ionsinto the implant area; annealing the semiconductor wafer to create atemporary region of dielectric in the semiconductor area beneath theimplanted area; applying a masking layer and performing a trench etchabove selected areas of the temporary region to at least the depth ofthe temporary region; and removing the temporary region to achieve thepredefined shape of the single crystal structure.
 2. The method of claim1 wherein the step of masking further comprises: applying a blockinglayer of photoresist to the first surface on the areas of the etch stoplayer where ion implantation is not desired.
 3. The method of claim 2further comprising the step of: removing the blocking layer ofphotoresist.
 4. The method of claim 1 wherein the step of ionimplantation further comprises: converting the semiconductor material toan oxide or a nitride below the single crystal material.
 5. The methodof claim 1 wherein the step of annealing further comprises: annealingthe semiconductor wafer to create a temporary region in thesemiconductor area beneath the implanted area to convert thesemiconductor area to an oxide or a nitride.
 6. The method of claim 1wherein the step of removing the temporary region to achieve thepredefined shape of the single crystal structure comprises: etching thesubsurface dielectric layer formed by the ion implantation step.
 7. Amethod of fabricating an air bridge, comprising the steps of: maskingthe first surface according to a predetermined shape to create animplant area on the first surface; implanting ions into the implantarea; annealing the semiconductor wafer to create a temporary region ofdielectric in the first epitaxial area beneath the implanted area;applying a masking layer and performing a trench etch above selectedareas of the temporary region to at least the depth of the temporaryregion; and removing the temporary region to achieve the predefinedshape of the single crystal structure.
 8. The method of claim 7 whereinthe step of masking further comprises: applying a blocking layer ofphotoresist to the first surface on the areas of the etch stop layerwhere ion implantation is not desired.
 9. The method of claim 8 furthercomprising the step of: removing the blocking layer of photoresist. 10.The method of claim 7 wherein the step of ion implantation furthercomprises: converting the semiconductor material to an oxide or anitride below the single crystal material.
 11. The method of claim 7wherein the step of annealing further comprises: annealing thesemiconductor wafer to create a temporary region in the semiconductorarea beneath the implanted area to convert the semiconductor area to anoxide or a nitride.
 12. The method of claim 7 wherein the step ofremoving the temporary region to achieve the predefined shape of thesingle crystal structure comprises: etching the subsurface dielectriclayer formed by ion implantation step.
 13. A method for fabricating asemiconductor region having a single crystal structure that is supportedat one or more locations, comprising the steps of: depositing anepitaxial layer on a first surface of a semiconductor wafer; masking thefirst surface according to a predetermined shape to create an implantarea on the first surface; implanting ions into a implant area;annealing the semiconductor wafer to create a temporary region in thefirst epitaxial area beneath the implanted area; applying a maskinglayer and performing a trench etch above selected areas of the temporaryregion to at least the depth of the temporary region; and removing thetemporary region to achieve the predefined shape of the single crystalstructure.
 14. The method of claim 13 wherein the step of maskingfurther comprises: applying a blocking layer of photoresist to the firstsurface on the areas of the etch stop layer where ion implantation isnot desired.
 15. The method of claim 14 further comprising the step of:removing the blocking layer of photoresist.
 16. The method of claim 13wherein the step of ion implantation further comprises: converting thesemiconductor material to an oxide or a nitride below the single crystalmaterial.
 17. The method of claim 13 wherein the step of annealingfurther comprises: annealing the semiconductor wafer to create atemporary region in the semiconductor area beneath the implanted area toconvert the semiconductor area to an oxide or a nitride.
 18. The methodof claim 13 wherein the step of removing the temporary region to achievethe predefined shape of the single crystal structure comprises: etchingthe subsurface dielectric layer formed by ion implantation step.
 19. Amethod of fabricating an air bridge, comprising the steps of: applying ablocking layer to the first surface; masking the first surface accordingto a predetermined shape to create an implant area on the first surface;implanting ions into the implant area; annealing the semiconductor waferto create a temporary region in the first area beneath the implantedarea; applying a masking layer and performing a trench etch aboveselected areas of the temporary region to at least the depth of thetemporary region; and removing the temporary region to achieve thepredefined shape of the single crystal structure.
 20. The method ofclaim 19 wherein the step of masking further comprises: applying ablocking layer of photoresist to the first surface on the areas of theetch stop layer where ion implantation is not desired.
 21. The method ofclaim 20 further comprising the step of: removing the blocking layer ofphotoresist.
 22. The method of claim 19 wherein the step of ionimplantation further comprises: converting the semiconductor material toan oxide or a nitride below the single crystal material.
 23. The methodof claim 19 wherein the step of annealing further comprises: annealingthe semiconductor wafer to create a temporary region in thesemiconductor area beneath the implanted area to convert thesemiconductor area to an oxide or a nitride.
 24. The method of claim 19further comprising the step of: depositing an additional epitaxiallayers on the semiconductor material to increase the single crystalthickness; implanting oxygen into this additional layer; annealing,after all implantation and etching, the annealed areas to allow thefabrication of multiple layer air bridges of single crystalsemiconductor material.
 25. The method of claim 19 further comprisingthe steps of: etching an opening through the temporary region and thesingle crystal semiconductor material: and filling it with conductivematerial.
 26. The method of claim 25 where the conductive material is ametal, metal alloy, a silicide, or doped polycrystalline semiconductormaterial.
 27. The method of claim 19 further comprising the step of:increasing the thickness of the single crystal layer by epitaxialdeposition.